Lewis Acid-Catalyzed [3+2] Cycloaddition of Donor-Acceptor Cyclopropanes and Enamines: Enantioselective Synthesis of Nitrogen-Functionalized Cyclopentane Derivatives

Article abstract of DOI:10.1002/adsc.201600221

A straightforward and efficient method for the synthesis of nitrogen-functionalized cyclopentane derivatives via [3+2] cycloaddition of enamines with donor-acceptor cyclopropanes in the presence of catalytic amounts of various Lewis acids at room temperat

Full text of DOI:10.1002/adsc.201600221

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DOI: 10.1002/adsc.201600221
Lewis Acid-Catalyzed [3+2] Cycloaddition of Donor-Acceptor
Cyclopropanes and Enamines: Enantioselective Synthesis of
Nitrogen-Functionalized Cyclopentane Derivatives
Kamal Vermaa and Prabal Banerjeea,*
a
Department of Chemistry, Indian Institute of Technology Ropar, Nangal Road, Rupnagar, Punjab – 140001, India
Fax : (+91)-1881-223395; e-mail: prabal@iitrpr.ac.in
Received: February 24, 2016; Revised: April 11, 2016; Published online: && &&, 0000
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201600221.
Abstract: A straightforward and efficient method
for the synthesis of nitrogen-functionalized cyclo-
pentane derivatives via [3+2] cycloaddition of en-
amines with donor-acceptor cyclopropanes in the
presence of catalytic amounts of various Lewis
acids at room temperature has been developed; fur-
thermore, the corresponding b-amino acid was syn-
thesized by monodecarboxylation and hydrogenoly-
Figure 1. Bioactive compounds containing NFC rings.
sis. An enantioenriched synthesis of nitrogen-func-
tionalized cyclopentane derivatives through dynam-
ic kinetic asymmetric transformation of racemic
donor-acceptor cyclopropanes has also been ach-
ieved employing a copper complex [Cu(OTf)2-L1]
as the catalyst affording an enantiomeric ratio up to
8:1.
annulations are known for the construction of cyclo-
pentane derivatives using DACs. These are (i) annula-
tion between various transition metal p-allyl activated
vinyl DACs and electron-deficient alkenes[6] and (ii)
Lewis acid-activated DACs and different alkenes.[7]
However, only a few reports are available in the liter-
ature wherein DACs have been used for the synthesis
of NFCs. The two major approaches used for the con-
struction of NFCs differ significantly by using differ-
ent substrates: (i) nitrogen functionality attached to
the cyclopropane moiety, and (ii) nitrogen functionali-
ty attached to the alkyne moiety (Scheme 1). The
Keywords: b-amino acids; donor-acceptor cyclopro-
panes; enamines; nitrogen-functionalized cyclopen-
tanes
Nitrogen-functionalized carbocycles are ubiquitously
found in numerous bioactive compounds relevant to
pharmaceutical interest.[1] In this context, the nitro-
gen-functionalized cyclopentane (NFC) ring is of par-
ticular importance (Figure 1). Cispentacin, an antifun-
gal antibiotic,[2] pactamycin, reported to act as an anti-
tumor, antiviral and antiprotozoal agent,[3] and pera-
mivir havꢀng potential for the treatment of H1N1 in-
fluenza,[4] are some illustrative examples of synthetic
drugs containing an NFC ring. Owing to their biologi-
cal activities, the development of a new synthetic pro-
tocol for NFCs is of greater interest.
It has widely been demonstrated that donor-accept-
or cyclopropanes (DACs) serve as all-carbon centered
1,3-zwitterionic synthons which, in the presence of
different Lewis acids, undergo [3+n] cycloadditions
to form cyclic skeletons.[5] Mainly two types of [3+2] Scheme 1. Synthesis of NFC derivatives.
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Table 1. Optimization of the reaction conditions.[a]
Waser group reported dynamic kinetic asymmetric
[3+2] annulations of N-succinimido-substituted cyclo-
propanes with both enol ethers and aldehydes using
a copper catalyst and commercially available BOX li-
gands. Excellent enantiomeric ratios (er) with good
yields of the NFCs containing a cyclopentyl or nucleo-
base have been achieved.[8] Johnson and his co-work-
ers reported an Sc(OTf)3-catalyzed [3+2] cycloaddi-
tion of DACs with ynamides to provide cyclopentene
sulfonamide derivatives.[9] Although excellent regiose-
lectivity with respect to ynamides was accomplished
in this report, the catalytic enantioselective version of
the titled transformation could not be achieved due to
the significant steric hindrance of the MgI2(PyBOX)
catalyst system. Herein, we report [3+2] cycloaddi-
tions of DACs with enamines to construct NFC deriv-
atives in the presence of various Lewis acids. An ap-
proach towards the asymmetric variant through dy-
namic kinetic asymmetric transformation (DyKAT)[10]
of racemic DAC 1b is presented by using the
Cu(OTf)2-L1 complex as catalyst, whereby an er up to
8:1 was achieved with good yield.
Entry LA
LA
(mol%)
Solvent
t
Yield [%][b] of
[h] 3ba/4ba
1
none
0
CH2Cl2 24 n.r[c]
2
3
4
5
MgI2
MgI2
MgI2
MgI2
MgI2
5
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
4
4
2
2
2
40/3:1
10
20
50
100
69/1.7:1
85/1.8:1
81/1.7:1
72/1.9:1
6
7
8
9
10
11
Cu(OTf)2 20
Sc(OTf)3 20
Sn(OTf)2 20
Yb(OTf)3 20
CH2Cl2 4.5 83/3:1
CH2Cl2 40 n.r[c]
CH2Cl2 40 n.r[c]
CH2Cl2 40 n.r[c]
CH2Cl2 40 n.r[c]
CH2Cl2 24 53/3:1
CH2Cl2 24 45/2.3:1
We started this investigation to explore the reaction
conditions that could effect the desired transforma-
tion (Table 1). Previously, we demonstrated that ring
InCl3
MgBr2
MgCl2
MgI2
MgI2
MgI2
MgI2
MgI2
20
20
20
20
20
20
20
20
opening reactions of DACs with other strained rings 12
13
(epoxides, aziridines) could best be achieved in the
presence of MgI2.[11] Based on our previous experien-
ces, initially we performed the reaction between 1b
and 2a in the presence of 5 mol% MgI2, which pro-
duced two separable diastereomers 3ba and 4ba in
a 3:1 ratio with a combined 40% yield (Table 1,
entry 2). Both 3ba and 4ba were characterized by
NMR spectroscopy. In addition, a single crystal X-ray
structure was sought for 3ba[12] (Figure 2, also see the
Supporting Information). Encouraged by this prelimi-
nary result, we next varied the loading of the catalyst
to improve the yield. The best yield of the products
was obtained when 20 mol% of MgI2 were used
(Table 1, entry 4). However, higher loadings (50 or
100 mol%) caused inferior results. To our delight,
Cu(OTf)2 shows a similar efficiency as that of MgI2
14[f]
15
DCE[d]
2
8
84/1.9:1
72/3:1
THF[e]
16
17
18
CH3CN 25 67/2.3:1
toluene 28 22/1.8:1
hexane 24 n.r[c]
[a]
All reactions were carried out under a nitrogen atmos-
phere: 1b (0.15 mmol, 1 equiv.), 2a (0.15 mmol, 1 equiv.),
Lewis acid (0.03 mmol, 0.2 equiv.), 4ꢂ MS (200 mg), sol-
vent (1 mL) at 258C.
Isolated yield after flash column chromatography.
n.r=no reaction.
DCE=1,2-dichloroethane.
THF=Tetrahydrofuran,
Temperature=808C.
[b]
[c]
[d]
[e]
[f]
producing the cycloadducts in excellent yield (Table 1, desired product (Table 1, entryies 8–11), MgCl2 and
entry 7). While other Lewis acids, such as Sc(OTf)3, MgBr2 gave moderate yields (Table 1, entries 12 and
Sn(OTf)2, Yb(OTf)3 and InCl3, failed to produce the 13). Various solvents were also explored in the course
of this optimization. A chlorinated solvent, such as
ClCH2CH2Cl or CH2Cl2 is found to be better than
non-chlorinated solvents (Table 1, entries 4 and 14).
While THF and acetonitrile provided moderate yields
(Table 1, entries 15 and 16).Toluene and hexane gave
low yield or no reaction, respectively (Table 1, en-
tries 17 and 18). Therefore, the optimal reaction con-
ditions for the cycloaddition were identified as
20 mol% of MgI2 or Cu(OTf)2 in CH2Cl2 at room
temperature.
With the optimized reaction conditions in hand, the
Figure 2. ORTEP structures of 3ba and 3be.
substrate scope was examined with a variety of DACs
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Table 2. Scope with respect to DACs.[a]
tially, we tested the methodology with more activated
DACs containing an electron-donating group, such as
methoxy (1a and 1b) and methylenedioxy (1c) present
in different positions of the aryl ring. Gratifyingly,
these variants of DACs gave the best yield of the
products 3aa & 4aa, 3ba & 4ba and 3ca & 4ca
(Table 2, entries 1–3). Notably, a conjugated cyclopen-
tenone 5 was observed in some cases (from cyclopro-
panes 1c and 1i), resulting from elimination of the
amine functionality. However, the reason behind this
specific elimination is not clear at this time. The 2,4,6-
trimethoxyphenyl substituted DAC 1d gave only
isomer 3da in 20 min (Table 2, entry 4). When a heter-
ocyclic ring was incorporated and the respective
DACs 1e and 1f were subjected to the above transfor-
mation, they also yielded the products 3ea & 4ea and
3fa (Table 2, entries 5 and 6). Noticeably, the more re-
active and highly substituted cyclopropanes 1d and 1f
each produce the single isomer. The yields of the re-
actions were suppressed when p-tolyl- and p-isopro-
pylphenyl-substituted DACs were used. Thus, a less
electron-donating alkyl group could cause a lower
yield of the reaction (Table 2, entries 7 and 8). Simple
phenyl-substituted 1k and o-methylphenyl-substituted
1j DACs failed to undergo the cycloaddition reactions
even at higher temperature (DCE, 808C). This may
be due to electronic and steric effect of the ortho sub-
stitution at the phenyl ring. To install an easily cleava-
ble group at NFCs, the reaction was tested with 2-
Entry DAC Ar
t [h] Products and Yields
[%][b]
3
4
5
1
2
3
4
5
6
7
8
9
1a
1b
1c
1d
1e
1f
1g
1h
1i
4-MeOC6H4
3,4-(MeO)2C6H3
3,4-(CH2O2)C6H3 2.5 58
2,4,6-(MeO)3C6H2 0.32 71
2-furyl
N-benzylindol-3-yl 0.32 68
4-MeC6H4
p-isopropylC6H4
(E)-CH=CHC6H5
3
2
61
55
19
30
15
–
–
À
9
–
3.5 56
22
–
–
–
7
54
6.5 38
22
14
–
35
–
–
23
8
[a]
All reactions were carried out under a nitrogen atmos-
phere: 1 (0.15 mmol, 1 equiv.), 2a (0.15 mmol, 1 equiv.),
MgI2 (0.03 mmol, 0.2 equiv.), 4ꢂ MS (200 mg), CH2Cl2
(1 mL) at 258C.
[b]
Isolated yield after flash column chromatography.
(1a–1l) as well as enamines (2a–2e) and the results styryl DAC 1i and the corresponding products 3ia, 4ia
are presented in Table 2 and Table 3.
& 5ia were obtained in high yield (Table 2, entry 9).
The reactivity of aryl-substituted DACs depends
The scope of the reaction was further extended by
upon the various groups present on the aryl ring. Ini- employing various b-aminoacrylates or enone 2b–2e
Table 3. Scope with respect to enamines.[a]
Entry
1
2
Ar
R1
R2
t [h]
Product and Yields [%][d]
3
4
1[b]
2[b]
3[b]
4[c]
5[c]
6[c]
7[c]
1b
1b
1b
1b
1l
1k
1g
2b
2c
2d
2e
2e
2e
2e
3,4-(MeO)2C6H3
3,4-(MeO)2C6H3
3,4-(MeO)2C6H3
3,4-(MeO)2C6H3
NPhTh
2-naphthyl
OMe
OMe
OMe
OMe
Me
4
2.4
3
2
2
50
84
82
88
82
61
63
–
–
–
–
–
14
16
-(CH2)2-
-(CH2)2O-
CH2Ph
CH2Ph
CH2Ph
CH2Ph
C6H5
4-MeC6H4
OMe
OMe
5
4.5
[a]
All reactions were carried out under a nitrogen atmosphere: 1 (0.15 mmol, 1 equiv.), 2 (0.15 mmol, 1 equiv.), Lewis acid
(0.03 mmol, 0.2 equiv.), 4ꢂ MS (200 mg), CH2Cl2 (1 mL) at 258C,
Lewis acid=MgI2,
Lewis acid=Cu(OTf)2,
Isolated yield after column chromatography.
[b]
[c]
[d]
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Scheme 2. Plausible mechanism.
with DAC 1b (Table 3). Reaction of 1b and dibenzyl-
ACHTUNGTRENNUNGamine-substituted enamine 2e in the presence of
Cu(OTf)2 gave only the single isomer 3be in 88% iso-
lated yield (Table 3, entry 4). The configuration of the
compound 3be was confirmed by NOE and single
crystal X-ray studies (Figure 2, see the Supporting In-
formation).[12] To have more nitrogen functionality in
the cycloadduct, 2e was also treated with N-contain-
Figure 3. NOE observations of products 3ba and 4ba.
ing cyclopropane 1l which gave 3le in 82% yield tion of 3 and 4 was confirmed by NOE spectroscopy
(Table 3, entry 5). We further examined the reaction techniques (see the Supporting Information and
of enamine 2e with less activated DACs. To our de- Figure 3).
light, phenyl- (1k) and p-methyl-substituted (1g)
Next, we became interested in examining the feasi-
DACs gave the corresponding diastereomeric mix- bility of performing an asymmetric variant of our
tures of the products 3ke & 4ke and 3ge & 4ge with methodology. Following the previous reports on
combined yields of 75% and 79%, respectively DyKAT,[13,8] we conducted the asymmetric reaction
(Table 3, entries 6 and 7). The enamines (1c or 1d) through DyKAT using racemic cyclopropane 1b with
that contain a pyrrolidine or morpholine amino func- enamine 2a in the presence of broad range of bisoxa-
tionality afforded 3bc and 3bd, respectively, in good zoline ligands (BOX) (L1–L3) in combination with
yields (Table 3, entries 2 and 3). When the phenyl Cu(OTf)2 (Table 4). Initially, we attempted the asym-
group was replaced by a naphthyl in enamine 2b, the metric reaction with racemic DAC 1b with 2a in the
yield of the reaction was decreased to 50% (Table 3, presence of Cu(OTf)2-L1 complex, which gives an er
entry 1).
of 8:1 and a 5.2:1 ratio of diastereomers 3ba and 4ba
Scheme 2 depicts a proposed mechanism of the re- with 78% yield (Table 4, entry 1). DAC 1b was fully
action between 1 and 2 using different Lewis acids. consumed in the reaction and yielded 78% of NFC
The Lewis acid activates the DAC ring and transforms products (3ba and 4ba). Compared to L1, the ligands
it to the intermediate A. Enamine 2 attacks on the in- L2 & L3 were found to give moderate er of both dia-
termediate A to generate the intermediate B. This in- stereomers (Table 4, entries 2 and 3). Other variants
termediate can adopt two possible conformations C of ligand were also studied for the above transforma-
and D. In conformer C, the two bulky groups repel tion for optimization (see the Supporting Informa-
each other and result in minor product 4. On the tion).
other hand, the conformer D featuring less steric hin-
To synthesize enantioenriched free amine-function-
drance leads to the major product 3. The configura- alized cyclopentane derivative, the reaction of 1b and
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Table 4. Asymmetric [3+2] cycloaddition reaction between
2a and 1b: Screening with various BOX ligands.[a]
Scheme 4. Decarboxylation and hydrogenolysis steps.
Entry L t [h] Yield [%][c] 3ba:4ba 3ba (er[b]) 4ba (er[b])
10% Pd/C afforded 7be with good yield of 82%
(Scheme 4).
1
2
3
L1 8
L2 9
L3 9
78
77
80
2.5:1
3:1
2.3:1
8:1
3.7:1
2.1:1
5.2:1
2.7:1
1.8:1
In conclusion, we have developed a [3+2] cycload-
dition reaction of enamines and DACs in the presence
of various Lewis acids, providing a workable access to
functionalized NFCs that are otherwise difficult to
prepare. We have also shown an effective synthesis of
enantiomerically pure NFC using the Cu(OTf)2-L1
catalytic system. Furthermore, decarboxylation and
hydrogenolysis of an NFC derivative to a cyclic b-
amino acid derivative may open up further synthetic
potential of this protocol. Studies on the further appli-
cations of this protocol to the synthesis of natural
products are in progress.
[a]
All reactions were carried out under a nitrogen atmos-
phere: 2a (0.15 mmol, 1 equiv.), 1b (0.15 mmol, 1 equiv.),
ligand (0.22 equiv.), Lewis acid (0.03 mmol, 0.2 equiv.),
4ꢂ MS (200 mg), CH2Cl2 (1 mL), room temperature.
er=enantiomeric ratio,
[b]
[c]
Isolated yield after flash column chromatography.
Experimental Section
General Procedure for the Cycloaddition Reaction
A two-necked round-bottom flask was charged with donor-
acceptor cyclopropane
1
(1.0 equiv.), enamines
2
(1.0 equiv.), 4ꢂ MS (200 mol%) and Lewis acid (0.2 equiv.)
under a nitrogen atmosphere. DCM was added to the reac-
tion mixture and solution was stirred at room temperature
until consumption of the cyclopropane (as monitored by
TLC). The reaction mixture was filtered through thin pad of
celite and solvent was concentrated in a rotary evaporator.
The residue was purified by flash column chromatography
on silica gel using ethyl acetate/hexane as eluent.
Scheme 3. Asymmetric synthesis of a cyclopentane deriva-
tive containing a free amine 8be.
2e in the presence of Cu(OTf)2-L1 complex as cata-
lyst yielded 3be, which on hydrogenolysis gave an
81% yield of 8be with 8:1 er (Scheme 3).
Acknowledgements
A synthetic application of the NFCs was demon-
strated further for the preparation of b-amino acids,
which have a great pharmacological potential.[14] An
NFC derivative 3be was partially decarboxylated in
the presence of KOH at 708C to give a single diaste-
KV thanks to the UGC, New Delhi, India for a research fel-
lowship. PB acknowledges the DST-India for financial sup-
port. The authors are also thankful to Dr. C. M. Nagaraja,
Department of Chemistry, IIT Ropar for solving the single
reoisomer 6be[15] and further hydrogenolysis using H2, crystal X-ray structure mentioned in this paper.
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Adv. Synth. Catal. 0000, 000, 0 – 0
6
ꢁ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÞÞ
These are not the final page numbers!

COMMUNICATIONS
Lewis Acid-Catalyzed [3+2] Cycloaddition of Donor-
Acceptor Cyclopropanes and Enamines: Enantioselective
Synthesis of Nitrogen-Functionalized Cyclopentane
Derivatives
7
Adv. Synth. Catal. 2016, 358, 1 – 7
Kamal Verma, Prabal Banerjee*
Adv. Synth. Catal. 0000, 000, 0 – 0
7
ꢁ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÞÞ
These are not the final page numbers!